The invention concerns a store and a method for large-scale storage of thermal energy in the form of high-temperature heat at a temperature level of at least 280° C.
Solar thermal power plants can produce electricity, heat or steam without a store only at times of high direct solar radiation. The higher the temperature of the heat that is provided by an array of concentrating solar collectors (in the following referred to as a solar field), the higher the efficiency of the connected power plants. In a seawater desalination plant there is also the need for solar-produced heat at high temperature.
In general, the solar thermal steam generator or the solar field are oversized so that already in the morning, a short time after sunrise, the appropriate steam quantities for generating electricity or for seawater desalination can be produced. Around noon, usually too much steam is produced so that mirror surfaces of the solar field are either folded away or the excess heat is thermally stored.
The heat storage can be realized either in short term stores which in general are embodied as steam stores, so-called Ruths stores, or in salt stores in which a liquid salt solution absorbs the heat to be stored. After sundown, the heat that is stored in the short term store is then released by means of a heat exchanger to the water/steam circulation of the steam power plant, a thermal oil circulation or a seawater desalination plant.
Up to now, there was no demand for large-scale heat stores at high-temperature level because the central application in a solar thermal power plant has been lacking in recent times, with few exceptions. In the meantime the number of solar thermal power plants under construction and going into operation, and thus the demand for reliable and inexpensive thermal stores with a great storage capacity increases worldwide more and more.
In case of concrete stores the bad heat transfer between concrete and the pipelines in which the heat carrier is circulated is a basic disadvantage.
PCM stores (“phase change material” stores) in principle represent very interesting storage systems. For high-temperature applications above 200° C., there are however only small pilot plants within the kilowatt range. In this connection, the heat release and absorption when changing the phase from liquid to solid and vice versa at the same temperature is used. The energy density is presumably higher than in concrete stores or salt stores. A disadvantage are the expected high costs for PCM stores as well as for the required metal pipes and surfaces for the heat transfer. A great problem is also charging and discharging of the store within acceptable time periods. Moreover, hysteresis effects have been observed in the small pilot stores that make the store less efficient and reduce the utilizable storage capacity. It is also still unclear to what extent PCM materials will survive unimpaired with respect to their efficiency the high numbers of charging and discharging cycles.
At the moment, appropriate salts for PCM stores are provided such as NaNO2, NaNO3, or KNO3. It is still unclear which materials can be used at higher temperatures above approximately 340° C. Celsius and at what costs. A disadvantage of PCM stores is the fixation to certain temperature levels, depending on which material is being used.
A great cost disadvantage of PCM stores, as in the case of a concrete store, are the numerous long and expensive high-pressure steam conduits that must be laid to the PCM store and must be extended through the PCM.
In the publication DT 24 44 217 dated Apr. 1, 1976, for example, it is discussed how sand or gravel can be used as a heat store when an appropriate conducting system is passed through the heat energy storage medium and which features the latter must fulfill. In large-scale practice, several thousand cubic meters of a storage medium sand or gravel are required for absorbing appropriate heat quantities. The proposed system of DT 24 44 217 is not realizable in practice for a large-scale application because, on the one hand, the costs of the pipeline system are too high and, on the other hand, as a result of the bad heat transfer between the pipeline system and the storage medium as well as within the storage medium, charging and discharging times are too long. Even for an optimized pipeline arrangement as disclosed in DT 24 44 217, the charging and discharging times are still too long. This system therefore is expected to have significantly worse operating properties in comparison to the above described concrete stores.
In the publication DE 27 31 115 dated Jan. 25, 1979, a system is described that uses a packed bed of granular solid particles and names natural magnesite or olivine as a storage material. This system is directed to smaller applications and has as a decisive feature that the charge flow circulates in a closed circuit. The heat in this system is not introduced by a heat exchanger but by electrical heating rods. This system is therefore not utilizable for large-scale applications with required storage capacities of much more than 20 MWh. If it were attempted to use this system in a large-scale variant, the problem is encountered that the charge flow over time will have an ever increasing temperature and the pressure in the closed charge circuit will increase. This requires a pressure-resistant design of the system and at the same time entails high costs. This effect of the temperature and pressure increase has also negative effects on the blower that circulates the charge flow. Moreover, charging of a thermal store for a solar thermal power plant by the detour of generating electricity and producing heat by means of heating rods is not sensible due to the efficiency loss for producing electricity.
U.S. Pat. No. 4,222,365 A discloses a heat store in which as a storage medium gravel or crushed rock, inter alia of granite or marble, is used.
The storage material is arranged together with a blower, a heat exchanger 48, and an air passage 56 within a housing 46. The heat exchanger 48 is flowed through by liquid alkali metal and releases heat therein to the air that is circulating in closed circulation within the housing. The air that is heated in this way flows through a bed of crushed rock which is heated thereby. Upon discharge of the store the flow direction of the air is reversed and the heat is from the crushed rock to the air and to the heat carrier that is flowing through the heat exchanger.
WO 2010/060524 discloses a heat store in which as a storage medium several solid bodies are used through which parallel passages extend. At the center of the heat store a heat exchanger is provided. By means of air that is circulating in a closed circuit within the heat store, the storage medium is charged and discharged.
A disadvantage of this system is the large outer dimensions of the housing as a result of the concept because within the housing, in addition to the actual storage medium, air passages, a blower and a heat exchanger must be arranged also. This increases the costs and reduces the efficiency of the store.
Moreover, the system disclosed in U.S. Pat. No. 4,222,365 A requires for discharging two heat exchangers (a first heat exchanger within the housing and a second heat exchanger in a steam generator in order to be able to use the stored heat in a steam turbine for producing electricity.
In principle, in the literature or in patent documents there is nothing to be found that meets the posed requirements with respect to high storage capacity at high temperatures with quick charging and discharging times and high cycle numbers. Modifications of related applications also doe not result in an acceptable solution.
In connection with the claimed invention, the following terminology is employed:
A high-temperature heat store is to be understood as one or several storage modules inclusive the corresponding peripheral devices (heat exchanger, blower, conduits, flaps etc.).
A storage module comprises a gas-tight housing having in its interior one or several storage media. These storage media can be charged or discharged via feed line(s) and discharge line(s). Since the storage modules according to the invention can be produced and sold independent of the peripheral devices, the instant application also claims the storage modules according to the invention as such.
The object of the invention is therefore to provide a storage system which comprises the actual high-temperature heat store and the correlated peripheral devices, that can be used at high temperatures and across wide temperature ranges, and in which the heat can be quickly introduced and also discharged again for several years with many charging and discharging cycles. At the same time, the storage system should require low investment and operating costs.